Known examples of hybrid-type power supply systems made by combining at least two types of power supply device that have different characteristics are power supply systems that supply DC electric power via an inverter (DC converter) to a three-phase AC motor constituting the motive power source of an electric vehicle or the like. One such type of power supply device used here is a power supply device that will be referred to in this specification as an ‘energy type’ power supply device which holds a large amount of energy and that permits electric power to be supplied stably and over long periods. The other type is a power supply device that will be referred to in this specification as a ‘power type’ power supply device which is capable of supplying and absorbing a large amount of power in step with sudden changes in the load such as those occurring during acceleration/deceleration. Examples of energy type devices include high capacity storage cells, fuel cells, and engine drive generators, while examples of power type devices include capacitors and hybrid cells and the like.
Hybrid power supply systems combine energy type devices and power type devices in order to even the burden on an energy type device as a result of a power type device adapting to load fluctuations during acceleration and deceleration, for example. FIGS. 1 to 3 show three kinds of constitution serving to represent conventionally known hybrid power supply systems.
The hybrid power supply system 1 shown in FIG. 1 has the most primitive constitution in which a storage cell 2 that typifies an energy type device and a capacitor 3 that typifies a power type device are simply connected in parallel. The substantially fixed voltage from the storage cell 2 is outputted to an inverter 4.
In this primitive hybrid power supply system 1, the voltage of the storage cell 2 is applied to the capacitor 3 as is. Here, the fluctuation width of the voltage of the storage cell 2 is small. For this reason, only a small amount of energy that corresponds with this small voltage fluctuation width can be supplied from the capacitor 3 to the inverter 4. In other words, the amount of energy stored in the capacitor 3 cannot be utilized effectively.
In the second-type hybrid power supply system 5 shown in FIG. 2, a storage cell 6 is connected to the inputs of a current-source type DC/DC converter 7, the outputs of the DC/DC converter 7 being connected in parallel to a capacitor 8. The supply of energy to the inverter 4 is performed by the capacitor 8, while the supply of energy to the capacitor 8 is performed by the cell 6 via the current-source type DC/DC converter 7. The output voltage to the inverter 4 is controlled so as to be substantially constant.
The second-type hybrid power supply system 5 shown in FIG. 2 affords the benefit that there is a large degree of freedom in the selection of the voltage of the storage cell 6. However, because the voltage fluctuation width of the storage cell 6 is small, there is naturally the problem that the energy stored in the capacitor 8 cannot be utilized effectively. However, because of the requirement for a DC/DC converter 7 that has an electrical capacitance equal to that of the inverter 4, the DC/DC converter 7 is large in size and the cost thereof is high. In addition, the efficiency drops in step with the electric power consumption of the DC/DC converter 7 which has this high electrical capacitance.
In the third-type hybrid power supply system 9 shown in FIG. 3, a capacitor 10 is connected to the inputs of a DC/DC converter 11, and the outputs of the DC/DC converter 11 are connected in parallel to a storage cell 12. The supply of energy to the inverter 4 is carried out by the capacitor 12, and steep energy supply and regeneration is performed by the capacitor 10 via the DC/DC converter 11. The output voltage to the inverter 4 is substantially constant.
With the third-type hybrid power supply system 9, there is the merit that the voltage of the capacitor 10 changes greatly and hence the stored energy of the capacitor 10 can be utilized effectively. However, because of the requirement for a DC/DC converter 11 that has an electrical capacitance equal to that of the inverter 4, the DC/DC converter 11 is large in size and the cost thereof is high. In addition, the efficiency drops in step with the electric power consumption of the DC/DC converter 11 which has this large electrical capacitance. Also, on account of the time lag of the electric power conversion by the DC/DC converter 11, the start of the discharge and absorption of a large current by the capacitor 10 is delayed.